AC power transmission lines are often protected by protective relay systems which trip circuit breakers to isolate a section of the lines if a fault is detected. The systems comprise local and remote terminals linked, via communication interfaces, by a communication channel along which information about the current, for example, is transmitted between the terminals. When this information indicates that a fault has occurred, the terminals trip the circuit breakers to isolate the line.
In the past, communication has taken place via two wires called pilot wires. It is now necessary to replace such wires by a four wire high speed digital channel communication system, but in doing this a problem has been encountered with the feedback of a received signal to the transmitting relay. It is an object of this invention to provide an interface in which such feedback is substantially reduced or eliminated.